1. Field of The Invention
The invention relates to optical fiber transmission cables and in particular to a new optical fiber cable and method and apparatus for making it.
2. Background Information
Optical fiber cables for transmission signals are known in the art. An optical fiber cable may be made up of a plurality of optical fibers housed within a sheath. An optical fiber is an elongated element of narrow diameter and great or indefinite length. The optical fiber is formed of glass or synthetic material and comprises a core of predetermined refractive index and a covering of smaller refractive index.
Use of optical fibers entails some difficulties, in that, large numbers of fibers must be contained in a single cable. The fibers must be placed in the cable such that each fiber is free from excessive mechanical stress which causes microbending in the fiber and a resultant increase in attenuation.
A preferred fiber optic cable utilizes fibers which propagate only one mode of light compared with multimode fibers which propagate several modes along the fiber core. Present single mode fibers are more sensitive to microbending losses, particularly at the longer wavelengths used in today's systems than multimode fibers previously used for telecommunications. The single mode fibers have inherent attenuation that is only a fraction of that in multimode fibers available a few years ago. Typically, telecommunication grade single mode fibers have an attenuation of 0.3 to 0.5 db/km at 1300 nm wavelengths as compared to multimode fibers which have attenuations of 3 to 4 db/km. Microbending can cause increased losses at this wavelength in the order of 0.1 to 0.3 db/km. Therefore any additional attenuation due to microbending is much more significant with signle mode fibers. Additionally, new transmission systems operate at 1550 nm on single mode fibers. Microbending at this wavelength can be in the order of 0.1 to 3.0 db/km which essentially could make the cable unusable for transmission at this wavelength. Therefore, reduced levels of stress and microbend induced attenuation are necessary for single mode cable designs.
In order to limit the attenuation various types of cables have been developed. U.S. Pat. No. 4,153,332 discloses an optical fiber cable wherein a group of tightly bundled optical fibers are housed within a tube and a plurality of those tubes are wound or stranded to produce a cable when covered by an outer sheath. This cable, however, is susceptible to water damage caused by water entering through a leak in the sheath and running along the interior space of the cable sheath and the tubes. Moreover, the fibers in the '332 cable are of unequal overall length. This is because the effective diameter of the cable coils differs depending on fiber location. That is, when the fibers are in a bundle, the fibers innermost in the coil travel a loop of smaller radius than do the fibers outermost in the coil.
U.S. Pat. No. 4,331,379 to Oestreich et al. provides for filling the space within the cable between a stranded fiber bundle and the inner cable wall with a soft filler. The filler improves the prevention of moisture ingress in the cable. However, the stranded inner bundle of optical fibers increases the bending stresses in the fibers and therefore increases the attenuation.
In manufacture, the stranded bundle is easier to handle, however, more machinery, and therefore increased cost is necessary to strand the fibers. If parallel fibers are used, they are often difficult to handle and the fibers may end up with widely divergent lengths and therefore inconsistent transmission quality.